The present invention relates to apparatus used to increase the efficiency of space heating systems by adjusting said heating system's operation based on a measurement of the conditions external to said space being heated.
Small scale heating systems (residential and most commercial), use conventional control systems with essentially no processing. For example, a conventional residential fossil fuel fired hydronic heating system operates by a simple on/off thermostat switch. The water temperature, then, must be set high enough to provide sufficient heating output under the worst case ambient temperature, say, -10.degree. F. When the ambient temperature is higher, the required BTU output is less, and the water temperature could be lowered, providing an energy savings through reduced standby and heat-up losses. Without some form of processing, however, this system must always run at maximum water temperature.
One concept that has been effectively applied to small heating systems is the reset principle. Systems which use said principle incorporate a temperature sensing device mounted externally to the structure being heated, to provide a measure of the external ambient temperature. The external temperature reading is used, based on a preset ratio of external ambient to heating system (furnace) temperature and heating system reference temperature, to adjust (i.e., reset) the temperature of the heating system fluid (usually water or air) for better heating system efficiency. Such systems are available commercially as additions to conventional heating system controllers, and many devices using variations on the application of the reset principle have had U.S. Patents granted. Whereas such systems are undoubtedly more energy efficient than simple conventional controllers, they cannot approach the efficiency of what we shall call herein an "ideal Reset" system due to the fact that they employ an open-loop temperature adjustment operation.
The Reset principle, as implemented in current systems and in prior U.S. Patent art, only accounts for one variable in the determination of a structure's heating load, albiet a primary one, external temperature. The external temperature determined from the system's sensor is used to automatically adjust the set temperature of the heating system's fluid (i.e., water) in order to minimize the operating temperature of the heating system in accordance with the heating load as determined by the external temperature. This, in turn, minimizes incidental heat loss at the heating system's furnace, providing an overall increase in the efficiency of the heating system.
Such implementation, however, does not account for other variables in the determination of heat load, such as wind, solar heating, and variations in structure insulation (e.g., removal or installation of storm windows, leaving a window open, etc.). The "ideal Reset" system is one that accounts for all variables in heating load determination in the process of computing the reset value for the heating system. Having accounted for all variables, the ideal Reset system will cause the fluid temperature of the heating system to be maintained at the most efficient, i.e. minimum, temperature possible for the present ambient conditions. This most efficient condition will manifest itself with the effect of the heating system operating (i.e., fluid circulating to the devices creating the exchange of heat) exactly 100% of the time, except when the external temperature is at or above the internal temperature of the structure. Due to the fact that the current Reset system implementations do not account for all variables, they must be set to operate with sufficient heat output to handle the worst case environmental conditions for a specific ambient temperature. Thus, such systems will generally not run at optimum efficiency, except for the infrequent situation where all worst-case conditions are met.
One other drawback to the currently available Reset systems is that manual adjustment of at least two parameters is required, either by the user or an installation technician, in order to put the device into service. These parameters are the Reset Ratio and the Reference Set Point. The Reset Ratio determines the ratio of the change in heating system fluid temperature that will be caused by a change in external ambient temperature. The Reference Set Point is usually selected as the Initial Set Point, that is, the heating system fluid temperature desired when the external ambient temperature is equal to the internal temperature (usually 70.degree. F.). The Reset Ratio is a very difficult parameter to determine, since it is based on factors such as heating system design, effective insulation of the structure, prevailing winds and exposure to the sun. This parameter can only be accurately determined by an expert versed in heating system design, insulation of structures, and other environmental effects on heat transfer. This parameter selection process is further complicated by the fact that in order to obtain an optimum reset curve at lower external temperatures, where heating is critical, the Initial Set Point may have to be modified from a simple nominal value. Finally, the selection of these parameters at the initial installation may be invalidated by subsequent modifications made by the structure owner, such as the installation of storm windows or added insulation.